This invention relates to conveyor belts and, more particularly, to modular plastic conveyor belts formed of rows of plastic belt modules pivotally interlinked by transverse pivot rods.
Because they do not corrode and are easy to clean, unlike metal conveyor belts, plastic conveyor belts are used widely, especially in conveying food products. Modular plastic conveyor belts are made up of molded plastic modular links, or belt modules, that can be arranged side by side in rows of selectable width. A series of spaced apart link ends extending from each side of the modules include aligned apertures to accommodate a pivot rod. The link ends along one end of a row of modules are intercalated with the link ends of an adjacent row. A pivot rod journalled in the aligned apertures of the side-by-side and end-to-end connected modules forms a hinge between adjacent rows. Rows of belt modules are connected together to form an endless conveyor belt capable of articulating about a drive sprocket.
In many industrial applications, conveyor belts are used to carry products along paths including curved segments. Belts capable of flexing sidewise to follow curved paths are referred to as side-flexing, turn, or radius belts. As a radius belt negotiates a turn, the belt must be able to fan out because the edge of the belt at the outside of the turn follows a longer path than the edge at the inside of the turn. In order to fan out, a modular plastic radius belt typically has provisions that allow it to collapse at the inside of a turn or to spread out at the outside of the turn.
Apertures slotted in the direction of travel of the belt are commonly provided in the link ends on at least one side of the modules to facilitate the collapsing and spreading of the belt.
The requirement of following a curved path causes problems not found in straight-running belts. As one example, radius belts, especially if tightly tensioned or running fast and lightly loaded, tend to rise out of the conveyor carryway around a turn. As another example, because belt pull is concentrated in the outer portion of the belt as it rounds a turn, outer link ends are more likely to fail unless otherwise strengthened or bolstered.
One modular plastic radius belt design is shown in U.S. Pat. Nos. 4,742,907 and 5,181,601. Various versions of the design include: a) integral guides depending from the belt and engaging a lateral surface of a supporting wearstrip at the outside of a turn to guide the belt around the turn; b) holddown tabs extending from the guides to hold the belt down as it rounds turns; c) heavy integral sideplates at the belt edge to withstand the increased stress experienced by the edge of the belt at the outside of a turn; and d) special high-strength, press-fit pivot rods.
Another modular plastic radius belt having internal modules with link ends of varying shapes is described in U.S. Pat. No. 5,174,439. The belt is driven off a curved drive bar central to each module. Special edge modules have closer link end spacing and tapered slots for pivot rods. Various rod retention schemes are shown, including plugging and press-fitting. Projections from the edge modules engage a side guide rail of the conveyor to prevent the belt from rising as it rounds a turn.
Conventional sprocket-driven conveyor belts include a drive surface engageable by the tooth of a sprocket about which the belt articulates. In many belts, such as the radius belts described in the previously mentioned patents, drive surfaces are formed along transverse elements disposed more or less midway between the link ends and connecting theta together. As a belt articulates about a sprocket, the teeth of the sprocket can rub against the drive surface as the tooth slides into and out of full engagement with the drive surface. This frictional rubbing, often referred to as scrubbing, causes wear on the drive surface and especially on the sprocket teeth. The problem of sprocket wear is often dodged through the use of beefy plastic sprockets or even metal sprockets.
Straight-running conveyor belts that are hinge-driven at a link end surface, rather than centrally driven along a surface between the link ends, are exemplified in U.S. Pat. Nos. 3,870,141 and 5,156,262. Neither belt, however, is capable of radius operation. The modules shown in the latter patent are hingedly interconnected by headless pivot rods. Belt edge structure can be flexed in and out of an occluding position restraining the pivot rod to permit its insertion or removal.
Because of the convenience of headless pivot rods, their use in conveyor belts is desirable. Many schemes for retaining headless rods include the use of retention clips that can be inserted and removed from a position occluding at least a portion of the aligned apertures. If such clips work their way out of their occluding position, they can contaminate the conveyed product or be lost. Other schemes for retaining headless rods are shown in U.S. Pat. No. 5,156,264. The techniques, however, require special rod treatment, such as tapering of the rod ends, or manual flexing of belt edge structure to admit the rod. Too much flexing of the belt edge structure can weaken the flexible joint.
There are other problems with some common belt designs. For example, stresses can be molded into the plastic modules during the molding process. Sharp, as opposed to curved, junctions between molded features on a belt module are more likely to form concentrated stress regions. When such modules make up a conveyor belt, operation of the belt increases the stress in those regions. In a radius belt, in which the pulling load is unevenly distributed across the width of the belt as it rounds a turn, the problem is exacerbated. One way to solve the problem is to add more material to the belt, but that makes the belt heavier and closes in some of the desirable open area that allows for drainage or air flow.
It is also advantageous to subject belt elements, especially protrusions such as link ends, to compressive rather than tensile forces, which tend to pull the elements apart. In the radius belts previously mentioned, consecutive link ends forming a pocket with the transverse drive element to accommodate a sprocket drive tooth are put in tension by the driving action of the tooth on the drive element.
Asymmetrical belts, especially belts having protrusions extending beyond the planes of the top and bottom belt surfaces, present handling problems. Such belts are not easy to roll up. Fewer linear feet of belt can be fit in a given box for shipment. The protrusions are more susceptible to damage, both during belt operation and in handling. An asymmetrical belt cannot be turned inside out (flipped top to bottom) to increase its useful life. Because asymmetrical belt modules are generally of a more irregular design, it is not so straightforward a process to cut them to specified widths for custom applications. Consequently, more molding tools are required and more module types must be kept in inventory.